Inductosyn scales which are electromagnetic induction type position detectors are used in position detection units of various machines such as machine tools, automobiles, and robots. The inductosyn scales include a linear scale and a rotary scale. The linear scale is installed in, for example, a linear moving portion such as a linear moving shaft of a machine tool and detects a linear moving position of the moving portion. The rotary scale is installed in, for example, a rotating portion such as a rotating shaft of a machine tool and detects a rotating position (rotating angle) of the rotating portion.
The linear scale and rotary scale both detect a position by use of electromagnetic induction in coil patterns which are arranged in parallel to face each other. The principle of this detection is described based on principle diagrams of FIGS. 2A to 2C.
FIG. 2A is a perspective view showing a state where a slider and a scale of the linear scale are arranged in parallel to face each other. FIG. 2B is a view showing the slider and the scale side by side. FIG. 2C is a view showing a degree of electromagnetic coupling between the slider and the scale.
Although, FIGS. 2A to 2C show the principle diagrams of the linear scale, the principle of the rotary scale is similar to this and a stator and a rotor of the rotary scale correspond respectively to the slider and the scale of the linear scale.
As shown in FIGS. 2A and 2B, a detection unit of the linear scale includes a slider 1 which is a primary-side member and a scale 2 which is a secondary-side member.
The slider 1 is a movable portion and includes a first slider coil 3 which is a first primary-side coil and a second slider coil 4 which is a second primary-side coil. The scale 2 is a fixed portion and includes a scale coil 5 which is a secondary-side coil. Each of these coils 3, 4, and 5 are formed to bend back and forth in a zigzag shape and to have a linear shape as a whole.
As shown in FIG. 2A, the slider 1 (the first slider coil 3 and the second slider coil 4) and the scale 2 (the scale coil 5) are arranged in parallel to face each other with a predetermined gap g maintained therebetween. Moreover, as shown in FIGS. 2A and 2B, the first slider coil 3 and the second slider coil 4 are shifted from each other by ¼ pitch.
In the linear scale, when an excitation current (an alternating current) flows through the first slider coil 3 and the second slider coil 4, as shown in FIG. 2C, the degree of electromagnetic coupling between the scale coil 5 and each of the first slider coil 3 and the second slider coil 4 changes periodically depending on change in relative position relationship between the scale coil 5 and each of the first slider coil 3 and the second slide coil 4 which is caused by the movement of the slider 1. Accordingly, an induced voltage which changes periodically is generated in the scale coil 5.
Specifically, a control unit of the linear scale causes a first excitation current Ia expressed by the following formula (1) to flow through the first slider coil 3 and causes a second excitation current Ib expressed by the following formula (2) to flow through the second slider coil 4.Ia=−I cos(kα)sin(ωt)  (1)Ib=I sin(kα)sin(ωt)  (2)
where                I: magnitudes of excitation currents        k: 2π/p        p: a value of one pitch of the coils (a length or an angle in the case of a rotary scale)        ω: an angular frequency of excitation currents (alternating currents)        t: a time point        α: an excitation position        
As a result, an induced voltage V expressed by the following formula (3) is generated in the scale coil 5 by electromagnetic induction between the scale coil 5 and the group of the first slider coil 3 and the second slider coil 4.V=KI sin(k(X−α))sin(ωt)  (3)
where                K: a coefficient of transfer depending on a gap g between the scale coil and the group of the first slider coil and the second slider coil and on the angular frequency ω of the excitation currents.        X: displacement of the detection unit (a moving position of the movable portion)        
The control unit receives the induced voltage V of the scale coil 5, calculates the value of the excitation position α at which the induced voltage V is equal to 0 (i.e. the excitation position α where X=α is satisfied), and outputs the thus-calculated excitation position α as the detected position X of the slider 1. Moreover, the control unit adjusts the first excitation current Ia and the second excitation current Ib on the basis of the thus-calculated excitation position α. Specifically, the control unit detects the position X of the slider 1 by performing control of satisfying the induced voltage V=0 by causing the position X of the slider 1 to follow the excitation position α to satisfy X=α, and then outputs the thus-detected position X.
Patent Literature 1 shown below is given as an example of a prior art document disclosing the electromagnetic induction type position detector. Patent Literature 1 describes, in claim 1 and the like, a technique in which a first detected position Xp and a second detected position Xm are obtained by using excitation currents Ia and Ib each having different angular frequencies and describes, in claim 3 and the like, a technique in which an alarm is outputted when the absolute value of the difference between Xp and Xm is larger than a threshold value.